The present invention relates to a predrawn net particularly for geotechnical use and to the method for manufacturing it.
Nets are already currently known which are obtained from initial components that have a considerable thickness and are constituted by flat symmetrical or non-symmetrical sheets provided with holes or recesses, which are then drawn, so as to obtain a net, in one or two directions, depending on whether the net is single- or double-drawn.
Nets of this type are disclosed for example in patents GB 2,073,090 and GB 2,035,191, that entail the formation of a flat sheet, with a square or rectangular pattern of holes, which is then drawn in one or two directions.
One of the problems that these patents seek to solve relates to the strength of the joints, which must be strong enough without containing too much plastic material; in order to solve this problem, production starts with an initial sheet-like component in which subsequent treatments produce scarcely drawn joints, since if the joints were to be drawn considerably, they would be particularly fragile if subjected to flexing transversely to the single-axis drawing direction.
In the case of double-drawn joints, the molecules that are at the center of the joint are either not stretched, and therefore do not effectively contribute to the strength of the entire structure, or are stretched radially with respect to the center of the joint, and accordingly the stretched molecules are not aligned with the direction of the stress applied to aligned filaments.
In this case the net, subjected to traction, can in fact break very easily at the joint, with imperfect utilization of the strength of the filaments.
For example, when single-drawn nets obtained by drawing a perforated sheet, according to the method disclosed in U.S. Pat. No. 4,374,798, are drawn substantially along the entire joint, they suffer easy breakage of said joint due to the application of transverse flexure or to the application of a concentrated load, such as the one produced by a rock in the soil.
If a rock produces a crack on a joint in products obtained from a flat perforated sheet, said products oppose no resistance to the spread of the crack throughout the joint, since said joint is orientated symmetrically.
Other known solutions, in which the initial product was constituted by two sets of filaments mutually superimposed and arranged transversely, generally had a very limited thickness, and in case of a relatively intense stretching the filaments of one set and the filaments of the other set would mutually separate in the region of the joint.
The alternative was therefore to stretch the filaments up to the inside of the joint, but not through the joints themselves, so as to avoid causing the separation of the filaments of the joint regions.